This invention relates to phase holograms and more particularly to the post-processing of dichromated gelatin holograms for correcting the Bragg angle deviation of the hologram.
An important factor in the determination of the relative merits of holograms is diffraction efficiency. With respect to the observation of a reconstructed image from holograms with the naked eye, a higher diffraction efficiency indicates that the laser output may be smaller for a predetermined level of brightness or that a brighter reconstructed image can be observed for a predetermined level of laser output intensity. A high diffraction efficiency of the hologram, however, has much greater significance, where holograms are used in optical scanning devices. More specifically, in a holographic scanner where a hologram is illuminated by a laser light beam to project the diffracted light beam along a scanning path, the laser must be compact in size, low in power consumption, long on life and inexpensive to manufacture. Therefore the intensity of the diffracted light for scanning an object must be higher to avoid the introduction of nonuniformity and low-output power for the scanning laser beam. Where silver-halide-gelatin holograms and dichromated-gelatin holograms are utilized in such scanning systems, the Bragg's angle deviation in such holograms is required to be corrected to zero with minimum diffraction efficiency lost during the treatment. The reconstruction angle of a hologram is one of the most important designa parameters in holographic scanners. The thickness of a volume phase gelatin hologram processed through a fast alcohol dehydration technique is normally increased. Because of this increased thickness, the reconstruction angle of volume phase holograms has a different value compared to the original construction angle. This has the effect of distorting the output laser beam thereby producing a very low efficency scanning system. This gelatin swelling property is the result of an incomplete orientational adsorption of water molecules to gelatin molecules in the fast dehydration process which is not stable in the presence of a water vapor environment. Prior art treatment for this type of hologram includes baking and vacuum processes to reduce the water content of the hologram. It has been found that this approach could not guarantee a complete correction of the Bragg's angle deviation. It is therefore a principal object of the invention to provide a method for post-processing a volume phase gelatin hologram for correcting the Bragg's angle deviation to a zero angle condition. It is another object of this invention to provide a method for post-processing a volume phase gelatin hologram for increasing the diffraction efficiency of the hologram. It is another object of this invention to produce a volume phase gelatin hologram that has a high diffraction efficiency and a high index of refraction modulation.